Off-set printing presses are widely used today. Stacked paper is placed in the feeder of the press and by the provision of an air blast and vacuum the top sheet is separated from the stack and fed into the press for printing. The entering sheet is directed between an impression cylinder and a blanket cylinder where the printing image is transferred from the blanket cylinder to the sheet of paper being carried between the impression cylinder and blanket cylinder. The sheet is then ejected into the delivery end of the printing press where the respective sheets are stacked one over the other.
It is important to appreciate that the above-described printing process is carried out at very high speeds. With such high speeds there is much opportunity for problems especially in the area of the final transfer and stacking of the printed sheets. In this regard, at the speeds in which the printing presses operate, it is difficult to effectively transfer and stack the printed sheets one over the other without the sheets curling and/or becoming misaligned.
More particularly and with respect to stacking, there are numerous factors and conditions that can affect the stacking ability of paper. Humidity, temperature, ink supply on the press rollers, water supply on the rollers, static electricity, operating speed of the press, paperweight type, and size and grain direction all can enter into stacking problems. Long grain paper has better stability because the grain itself holds the sheet out. Short grain paper has the least ability because its strength is applied only to the shortest distance. Long grain paper is of course preferred but is not always available. But even so, long grain paper can fall victim to the elements mentioned above but not to a degree as short grain paper.
In addition, the friction between the respective sheets tends to inhibit the quick and orderly transfer of respective sheets. Also in certain presses the ink is still wet during stacking and this gives rise to the transfer of ink from the top of one sheet to the back of an overlying sheet. Thus there is a need in such printing presses to improve the movement efficiency of the respective sheets and to prohibit the transfer of ink from one sheet to another.